The present invention relates to an objective optical system for an optical information recording/reproducing apparatus configured to suitably record information to and/or reproduce information from a plurality of types of optical discs based on different standards, and to an optical information recording/reproducing apparatus on which such an objective optical system is mounted.
There exist various standards of optical discs, such as DVD (Digital Versatile Disc) and BD (Blu-ray Disc), differing in recording density, protective layer thickness, etc. Therefore, an objective optical system mounted on the optical information recording/reproducing apparatus is required to have a compatibility with a plurality of types of optical discs. In this case, the term “compatibility” means to guarantee realizing information recording and information reproducing without the need for replacement of components even when the optical disc being used is changed. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording.
In order to have the compatibility with the plurality of types of optical discs based on the different standards, it is necessary to correct the spherical aberration which changes depending on the difference in protective layer thickness (i.e., a distance between a recording surface and a surface of a protective layer on an optical disc) between the optical discs and to form a suitable beam spot in accordance with the difference in recording density between the optical discs by changing the numerical aperture NA for information recording or information reproducing. In general, an optical information recording/reproducing apparatus is configured to selectively use one of a plurality of laser beams having different wavelengths depending on the recording density of the optical disc being used. The optical information recording/reproducing apparatus uses, for example, light having the wavelength of approximately 790 nm (i.e., so-called near-infrared laser light) for information recording or information reproducing for CD, light having the wavelength of approximately 660 nm (i.e., so-called red laser light) for information recording or information reproducing for DVD and light having the wavelength of approximately 405 nm (i.e., so-called blue laser light) for information recording or information reproducing for BD.
Each of Japanese Patent Provisional Publication No. 2006-164498A (hereafter, referred to as JP2006-164498A) and Published International Patent Application WO2008/007552 discloses a concrete example of an optical information recording/reproducing apparatus having the compatibility with three types of optical discs.
The optical information recording/reproducing apparatus disclosed in JP2006-164498A has an objective optical system including an optical surface on which different diffraction structures are provided in respective regions so as to enhance the light use efficiency for each laser beam. The diffraction structures are formed such that the diffraction efficiency is low in an inner region and is high in an outer region. Therefore, a beam spot formed on a recording surface of an optical disc is reduced to a level considerably smaller than an expected size due to the super-resolution, and the side lobe increases. For this reason, there is a concern that the information recording or the information reproducing might not be suitably performed.
In the optical information recording/reproducing apparatus disclosed in WO2008/007552, the diffraction efficiency during use of the blue laser light is low in an inner region and is high in an outer region. Therefore, as in the case of JP2006-164498A, the spot size is reduced excessively and the side lobe increases. In this case, the information recording or the information reproducing for an optical disc can not be suitably performed.